The speed of data communication is being pushed to ever increasing rates. The advent of Digital Subscriber Lines (DSL) now makes data communication in the megabit-per-second range possible across existing copper wire links between subscriber modems and central office modems in the public switched telephone network (PSTN).
As known to those skilled in the art, a central office provides individual subscribers with access to the PSTN. In most cases, a subscriber is linked to the central office via a twisted pair of copper wires. The central office provides an interface between the subscriber to the PSTN.
In order to facilitate DSL communication, a DSL modem is included in the link at the central office to communicate with a DSL modem used on the subscriber end of the two wire pair. DSL provides high-speed multimedia services which can operate hundreds of times faster than traditional analog telephone modems.
DSL comes in several different configurations. One is the Asymmetric Digital Subscriber Line (ADSL) which provides data rates of 32 kbps to 8.192 Mbps, while simultaneously providing telephone phone service. Also, Rate Adaptive Asymmetric Digital Subscriber Line (RADSL) is much like ADSL, only it allows bandwidth adjustment to fit the particular application and to accommodate the length and quality of the line. In particular, the data rate of a RADSL may be adjusted downward to accommodate a longer distance to the central office. Other configurations include High-bit-rate Digital Subscriber Line (HDSL), Symmetric Digital Subscriber Line (SDSL), and Very high-bit-rate Digital Subscriber Line (VDSL).
While DSL provides much higher rates of data communication, it is not without problems. In particular, at the higher frequencies used in DSL communication, the traditional two wire interface may not provide a reliable pathway or channel through which the data signal can travel. Often times, an interfering signal may be induced onto the two wire channel from a second two wire in close channel proximity. Such a signal might be from a second DSL modem that is communicating to the same central office.
Another problem may be the quality of the two wire channel itself. High frequency traffic generally experiences greater attenuation in the two wire channels. Also, interconnections that occur in the channel may degrade or loosen over a period of time, causing noise and further signal degradation.
As a result, data communication using DSL is becoming more susceptible to interference that causes disruption of the data signal. As more and more digital subscriber lines are installed, the probability of interference among two wire channels increases. Also, as the existing copper two wire network gets older, the quality of the channels will further degrade. Additionally, as DSL gains in popularity, it is likely that subscribers who are located greater distances away from central offices will desire DSL service due to its faster data communication rates. However, the longer distance results in greater signal attenuation as there is more opportunity for degradation, interference, and disruption of the data signal.
All of these problems may affect the quality of DSL signals transmitted across the two wire pairs. In particular, the information sent from which timing recovery of the data signal is determined at a particular center frequency in the receiving modem may be diminished or lost. However, it is now possible to send timing recovery information across a broad frequency spectrum in which timing recovery information may be recovered at any number of center frequencies along the spectrum.
However, the problems of signal degradation, interference, and disruption may compromise the timing recovery information received at any one of the number of center frequencies along the transmission frequency spectrum. In such a case, timing recovery cannot be established at such center frequencies and the receiver will have to randomly search through several possible center frequencies until timing recovery is established.
This hit and miss approach will cause an intolerable delay in the startup of data communication. Consequently, there is a need for a DSL receiver which can quickly evaluate timing recovery information received at various center frequencies to determine whether timing recovery may be reliably established.